Referring to FIG. 1, my basic desulfurization process can be described as illustrated therein. Flue gas from fuel-burning furnaces or industrial operations is treated in order to separate solid particles such as fly ash. The gas is then passed to a catalytic converter where a large fraction of the sulfur dioxide in the gas is oxidized into sulfur trioxide. The gas then enters a fluidized lime reactor where the sulfur dioxide/sulfur trioxide mixture is allowed to react with lime particles. The resulting solid material of the dry scrubbing is called "Linfan", the bulk of which can be separated from the gas and recovered as a valuable byproduct. The remaining dust in the exit gas is removed by cyclones or other suitable devices located downstream of the fluid bed reactor. The final gaseous effluent leaving the desulfurization process is essentially free of dust and sulfur oxides.
The solid byproduct ("Linfan") contains lime in its central core surrounded by an outer cracked shell of anhydrous calcium sulfate (CaSO.sub.4). Unlike other byproduct generated in dry processes, this outer shell has numerous micropores (very fine cracks) which are actually unique to my process. This byproduct has been shown to be useful for construction materials, chemicals for water and waste-water treatment, or even as a sorbent for sulfur dioxide scrubbing processes.
Since my desulfurization process involves a catalytic oxidation conversion as a gas preconditioning step, concerns have been raised regarding any potential operational problems. As far as solids removal and catalytic conversion are concerned, my desulfurization process is similar to Monsanto's Cat-Ox process. The article entitled The Final Report, Demonstration/Evaluation of the Cat-Ox Flue Gas Desulfurization, EPA-600/2-78-063 states the result of using vanadium pentoxide as a catalyst. It shows that: (1) flue gas has no poisoning effect on the vanadium pentoxide catalyst, (2) fly ash build-up has little affect on the conversion efficiency of the catalyst, and (3) the catalyst bed can be cleaned on a continuous basis, e.g., the catalyst may be shifted once every three months to minimize pressure drops across the bed.
The effort to develop my desulfurization process was first initiated in the early 1970's. The invention has been patented and reports describing various aspects of this process have been issued. The report published by the national Technical Information Services (NTIS) and entitled "Project on Sulfur Dioxide Removal and Waste Products Utilization Process", DOE/CE/15143-T1 (DE 84017333) contains information on the test results from a pilot plant operation of my process. Table 1 prepared from the information in that report is listed below.
TABLE 1 ______________________________________ Test Results of SOx Removal by Lime in the Fluidized Lime Reactor of Lin's Desulfurization Process SOx Reactor Conc.* SO.sub.2 Conc. SOx Temper- Reactor Reactor Reactor SO.sub.2 /SO.sub.3 Removal ature Inlet Inlet Outlet Ratio at Efficiency .degree.F. (ppm) (ppm) (ppm) Inlet (%) ______________________________________ 652 5000 2200 200 0.44 96 505 10000 500 0 0.05 100 709 20000 5400 200 0.27 99 540 10000 500 0 0.05 100 740 20000 1400 0 0.07 100 590 10000 4500 200 0.45 98 672 10000 4400 150 0.44 98.5 831 10000 500 0 0.05 100 739 5000 250 0 0.05 100 905 5000 200 0 0.05 100 ______________________________________ *No SO.sub.3 was detected at reactor outlet.
From the results contained in Table 1, it can be concluded that the desulfurization process is effective in removing nearly 100% of sulfur oxides from flue gas at temperatures ranging from 500.degree. F. to 900.degree. F., that sulfur oxide can be effectively removed by lime when sulfur trioxide is present in the flue gas, and that sulfur dioxide can be effectively removed by lime at a temperature as low as 505.degree. F. (263.degree. C.) when sulfur trioxide is present in the flue gas.
I have now determined that the effectiveness of my desulfurization process in removing sulfur dioxide from flue gas is largely due to the presence of sulfur trioxide in the gas. The SO.sub.x removal mechanism in my desulfurization process can be explained in the following manner. As the flue gas leaves the catalytic oxidation convertor, SO.sub.2 and SO.sub.3 in the gas are in equilibrium in accordance with the equation: EQU SO.sub.2 +1/2O.sub.2 .revreaction.SO.sub.3 ( 1)
However, after the flue gas containing sulfur dioxide and sulfur trioxide enters the lime reactor (located downstream of the catalytic converter), sulfur trioxide immediately reacts with lime (CaO) to form anhydrous calcium sulfate (CaSO.sub.4) according to the following formula: EQU SO.sub.3 +CaO.fwdarw.CaSO.sub.4, .DELTA.H=-9.26 K cal/mole(2)
This reaction is highly irreversible since the dissociation temperature of CaSO.sub.4 is known to be very high, about 2300.degree. F. Also, because the large amount of heat generated from the chemical reaction is rapidly dissipated in a fluidized environment, the reaction in equation (2) is enhanced. As a result of the rapid removal of sulfur trioxide, the equilibrium of equation (1) is destroyed, and the reaction direction is shifted completely to the right. Therefore, sulfur trioxide and CaSO.sub.4 are formed at an unusual speed: EQU SO.sub.2 +1/2O.sub.2 .fwdarw.SO.sub.3 ( 3) EQU SO.sub.3 +CaO.fwdarw.CaSO.sub.4 ( 2)
The reaction rate of equation (3) depends on oxygen concentration and the temperature of the flue gas. The higher the temperature of the reactor environment, the faster the reaction rate. Also, the higher the oxygen concentration of the flue gas, the faster the conversion rate. The reaction rate for sulfur trioxide formation under normal conditions can be calculated as follows: EQU d[SO.sub.3 ]/dt=k.sub.1 [O][M]{[St]-[SO.sub.3 ]}-{k.sub.2 [O][M]+k.sub.3 [H]}[SO.sub.3 ] (4)
where
[O] is oxygen concentration
[M] is background molecular concentration
[St] is total sulfur oxides concentration
Because sulfur trioxide reacts rapidly with calcium oxide to form CaSO.sub.4 in my desulfurization system, the sulfur trioxide in the second term of the equation (4) can be treated as zero and the [SO.sub.2 ] can be treated as [St] in the gas immediately surrounding the lime particle, the above equation becomes: EQU d[SO.sub.3 ]/dt=k.sub.1 [SO.sub.2 ][O][M] (5)
Westenberg and de Hass (J. Chem. Phys., 63, 5411-5415, 1975) recommended a rate coefficient of EQU k.sub.1 =8.0.times.10.sup.4 e.sup.-1400/T m.sup.6 mol.sup.-2 s.sup.-1( 6)
The value of K.sub.1 in equation (6) was determined under controlled laboratory conditions, i.e., homogeneous and steady state; the influencing factors such as fluidization, gas convection, and heat generated from the exothermic reactions, being important in my desulfurization system, were not included, as considerations. It is obvious that the conversion rate of SO.sub.2 to SO.sub.3 in my system is much faster than that predicted by equation (6). Therefore, the reaction rate values derived from equation (6) can only serve as a reference for my reaction mechanism.
From equations (4) and (5), it can be seen that the rate of SO.sub.2 conversion to SO.sub.3 in my desulfurization system is affected by oxygen concentration, background molecular concentration, SO.sub.2 concentration, and rate coefficient k.sub.1 which is a function of temperature. If the reaction time in the reactor is sufficiently long, all of the SO.sub.2 can be changed to SO.sub.3 which reacts in turn instantly with CaO and is removed from the flue gas. In reality, the reaction time in the lime reactor is very short, being a fraction of second or seconds.
In the lime reactor, the background molecular concentration is not affected by the chemical reactions. For gas with low SO.sub.2 concentration such as in a power plant, the reduction of oxygen concentration for desulfurization of the flue gas is not significant. Therefore, the SO.sub.2 conversion rate is mainly determined by rate coefficient k.sub.1 or temperature.
The average gas temperature of the fluidized lime reactor in my system, for economic reasons, is relatively low as compared with other processes. The reason why my process has such high SOx removal efficiency can be explained as follows: In a fluidized reactor environment, the lime particle is surrounded by a boundary layer which consists of a laminar sublayer. Referring to FIG. 1A, as an SO.sub.3 molecule reacts with lime at the points of reaction 1 , a large quantity of heat in the amount of 92.3 k-cal/mole is generated. According to calculations based on an average heat capacity of the reactant lime-bearing material of 0.26 cal/gm/.degree. C, if there is no heat transfer, the temperature at reaction points 1 is increased instantly by 2610.degree. C. or 4730.degree. F. Actually, the heat generated is transmitted immediately to the other part of the solid particle and to the surrounding gas. Since the heat transmission through the solid is relatively slow, there is a gradual temperature increase of the lime-bearing particle until thermal balance of the particle is attained. In a fluidized environment most of the heat is rapidly transmitted to the surrounding gas mainly by radiation and convection. Instantly, there is a sharp gas thermal gradient which induces violet turbulence of the gas flow on the surface of the lime particle. Thus, the boundary layer including the laminar sublayer of the particle is temporarily modified or destroyed. Because of the thermal induced turbulence and gas diffusion, more SOx is brought in contact with lime, and further reaction is enhanced.
When SO.sub.3 concentration of the flue gas in the reactor is high, the chemical reaction between lime and SO.sub.3 takes place continuously on the general surface of the lime-bearing particle. As a result, a layer of CaSO.sub.4 coating is formed on the surface of the lime-bearing particle. Subsequently, SO.sub.3 must diffuse through the CaSO.sub.4 coating in order to react with the lime in the core of the particle. The heat generated from the reaction will result in a violent convective turbulence on the whole surface of the particle. It can be visualized that the lime bearing particle 7 is surrounded by a chemically reacting gas layer 2 as shown in FIG. 1B. The chemically reacting layer 2 has a thin thickness and a sharp temperature gradient, ranging from high temperature at gas/solid interface 3 to about 1000.degree. F. at the edge of the layer 4. The maximum temperature at gas/solid interface 3 can be as high as 2300.degree. F., the dissociate temperature of CaSO.sub.4. The high gas temperature and gas turbulence near the interface 3 cause an increase in the kinetic energy of the gas molecules and high frequency of molecular collision. Consequently, the SO.sub.2 molecules are activated and rapidly oxidized to SO.sub.3. The newly formed SO.sub.3 also diffuses through the CaSO.sub.4 coating, reacts with lime 6 and contributes to the formation of CaSO.sub.4 coating 5 on the lime-bearing particle 7. In the mean time, because of the high temperature gradient of the chemically reacting layer 2, the relatively cold gases in the reactor are drawn into the layer and forced rapidly toward gas/solid interface 3. As SO.sub.3 in the gas is continuously diffused through the CaSO.sub.4 coating and removed by lime 6 in the lime-bearing particle, there exists an SOx concentration gradient in the chemically reacting layer 2. The SOx concentration gradient further induces SOx diffusion in the layer 2. Obviously, the movement of SOx molecules in the chemically reacting layer is very fast and its general direction is toward the lime-bearing particle. The characteristics of the chemically reacting gas layer 2 are that the reaction rate is non-equilibrium, the flow is turbulent, unsteady and three dimensional, and the condition is non-homogeneous.
The SO.sub.3 at gas/solid interface continuously diffuses through the CaSO.sub.4 coating and reacts with lime at the core of the lime-bearing particle 7. The diffusion rate of sulfur trioxide in CaSO.sub.4 coating is dependent on the temperature of the lime-bearing particle 7. On account of the highly exothermic reaction between sulfur trioxide and lime, the temperature of the lime bearing particle 7 gradually increases to a steady high level at which the heat released from further chemical reaction just balances with the heat dissipated to the fluidized reactor environment. The final steady temperature level of the lime bearing particle 7 as a whole increases with the SOx concentration of the influent flue gas and the average gas temperature of the reactor environment. It is understood that the higher the temperature of the lime-bearing particle, the higher the lime utilization efficiency, as long as the temperature does not exceed the dissociate temperature of CaSO.sub.4. Since the formation of CaSO.sub.4 in a Linfan particle takes place at very high temperature on account of the exothermic reaction, the CaSO.sub.4 is hard burned anhydrous CaSO.sub.4 (a new material).
The SOx removal efficiency of my process depends on several factors, namely, original SO.sub.3 concentration of the flue gas just before entering the fluidized lime reactor, the average gas temperature in the reactor, and residence time or reaction time of the gas in the reactor. For practical considerations, the reaction time in the lime reactor varies from 1/2 second to 10 seconds, the superficial velocity of the flue gas in the reactor is in the range from 1 foot per second to 80 feet per second, and the SO.sub.2 /SOx ratio can be more than 50%.
Flue gas contains water, therefore, the following reactions also take place: EQU SO.sub.2 +1/2O.sub.2 .revreaction.SO.sub.3 ( 1) EQU SO.sub.3 +H.sub.2 O.fwdarw.H.sub.2 SO.sub.4 ( 9)
The formation of H.sub.2 SO.sub.4 vapor in equation (9) removes SO.sub.3 from the flue gas, and thus, causes equation (1) to shift to the right direction, resulting in more SO.sub.3 production in the flue gas. Equation (9) is not significant in a high temperature environment. As the H.sub.2 SO.sub.4 vapor containing gas diffuses into chemically reacting layer 2 of the lime-bearing particle 7 (see FIG. 1B), the high temperature of the gas near the gas/solid interface 3 causes the H.sub.2 SO.sub.4 vapor to dissociate to SO.sub.3 and H.sub.2 O vapor. The SO.sub.3 molecules are removed immediately by the lime in the lime-bearing particle according to the following formula:
H.sub.2 SO.sub.4 .fwdarw.SO.sub.3 +H.sub.2 O (10) EQU SO.sub.3 +CaO.fwdarw.CaSO.sub.4 ( 2)
From equations (1), (9), (10), and (2), it is apparent that water vapor in flue gas can serve as a catalyst in the conversion of SO.sub.2 to SO.sub.3 in Lin's flue gas desulfurization process.
Flue gas may contain carbon dioxide, and the following reactions also take place in the fluidized reactor: EQU CO.sub.2 +CaO.fwdarw.CaCO.sub.3 ( 7) EQU SO.sub.2 +1/2O.sub.2 .fwdarw.SO.sub.3 ( 2) EQU SO.sub.3 +CaO.fwdarw.CaSO.sub.4 ( 3) EQU SO.sub.3 +CaCO.sub.3 .fwdarw.CaSO.sub.4 +CO.sub.2 ( 8)
In a fluidizing environment, equations (2), (3), (7) and (8) are irreversible reactions in my desulfurization process. Obviously, sulfur dioxide is continuously and effectively removed from the flue gas, and the resulting product of the desulfurization is mainly CaSO.sub.4. The reaction rates of equation (2), (3) and (8) are very fast in my process. Experience shows that either bubbling or circulating fluidize lime reactors can be employed for the effective removal of sulfur dioxide.
It should be pointed out that although a direct reaction between sulfur dioxide and calcium oxide may appear to be possible, this reaction has been shown to be absent in my desulfurization process. This fact can be explained by the findings of basic sulfur dioxide/calcium oxide reactions by Boyton and Chan et al.
Boyton in his book Chemistry and Technology of Lime and Limestone, Intersciences, states that adsorption of sulfur dioxide by lime occurs at a temperature between 300.degree. C. and 400.degree. C. At a temperature above 400.degree. C., sulfur dioxide reacts rapidly with lime to form CaSO.sub.3 according to the formula: EQU SO.sub.2 +CaO.fwdarw.CaSO.sub.3
At a higher temperature, the resulting products from the chemical reaction between lime and sulfur dioxide are CaSO.sub.4, CaSO.sub.3, and CaS. According to Chan et al in Thermogravimetric Analysis of Ontario Limestones and Dolomites, and in Reactivity of Sulfur Dioxide with Calcined Samples, by R.K. Chan et al., Canadian Journal of Chemistry, Vol. 48, No. 19, 1970, the extent of these three products depends on the reaction temperature.
There is no evidence that direct reaction between sulfur dioxide and calcium oxide takes place when sulfur trioxide is present in the lime reactor. A recent analysis of the resulting product of the desulfurization from a fluidized lime reactor shows that the product does not contain any trace of CaSO.sub.3 and CaS. The fact that CaSO.sub.3 and CaS are absent in the resulting product is an indication that direct reaction between sulfur dioxide and calcium oxide does not take place. Furthermore, as shown in Table 1, even at a reactor temperature as low as 500.degree. F. (263.degree. C.), sulfur dioxide can be removed effectively by lime. At such a low temperature, according to Boyton, there is simply no direct reaction between sulfur dioxide and calcium oxide. This is an indication that the reaction of my desulfurization process is mainly due to the presence of sulfur trioxide so that sulfur dioxide is facilitated in its conversion to sulfur trioxide for reaction with calcium oxide as explained by the aforementioned reaction mechanism.
Apparently, the resulting product of my desulfurization process contains only unspent lime and anhydrous CaSO.sub.4 which is produced from a reaction between sulfur trioxide and calcium oxide. Because it is relatively pure, it has commercial value.